According to the cyclic evolutionary model, ices evolve
chemically and physically in interstellar space, so do the organics.
Where and how the interstellar dust is formed appears to
involve a complex evolutionary picture. The rates of
production of refractory components such as silicates
in stars do not seem to be able to provide more than
about 10% of what is observed in space because they
are competing with destruction which is about 10 times
faster by, generally, supernova shocks
(Draine & Salpeter 1979a,
b;
Jones et al. 1994).
At present the only way to account for the observed
extinction amount is to resupply the dust by
processes which occur in the interstellar medium
itself. The organic mantles on the silicate particles must
be created at a rate sufficient to balance their
destruction. Furthermore, they provide a shield against
destruction of the silicates. Without them the silicates would indeed be
underabundant unless most of the grain mass was condensed
in the ISM, as suggested by
Draine (1990).

What is currently known about the organic dust component
is based very largely on results of laboratory experiments
which attempt to simulate interstellar processes.
The organic refractories which are derived from
the photoprocessing of ices contain a mixture of aliphatic
and aromatic carbonaceous molecules
(Greenberg et al. 2000).
The laboratory analog suggests the presence of abundant
prebiotic organic molecules in interstellar dust
(Briggs et al. 1992).

The silicate core-organic mantle model is recently revisited
by Li & Greenberg (1997)
in terms of a trimodal size
distribution consisting of (1) large core-mantle grains
which account for the interstellar polarization,
the visual/near-IR extinction, and the far-IR emission;
(2) small carbonaceous grains of graphitic nature to
produce the 2175 Å extinction hump;
(3) polycyclic aromatic hydrocarbons (PAHs) to
account for the far-UV extinction as well as the
observed near- and mid-IR emission features at
3.3, 6.2, 7.7, 8.6, and 11.3 µm. This model is
able to reproduce both the interstellar extinction
and linear and circular polarization.

Very recently, Draine and his co-workers
(Li & Draine 2001b,
2002a;
Weingartner & Draine
2001a)
have extended the silicate/graphite grain model
to explicitly include a PAH component as
the small-size end of the carbonaceous grain population.
The silicate/graphite-PAHs model provides an excellent
quantitative agreement with the observations of IR emission
as well as extinction from the diffuse ISM of the Milky Way Galaxy
and the Small Magellanic Cloud.

Mathis & Whiffen (1989)
have proposed that interstellar
grains are composite collections of small silicates,
vacuum ( 80% in volume),
and carbon of various kinds (amorphous carbon, hydrogenated
amorphous carbon, organic refractories).
However, the composite grains may be too cold and produce
too flat a far-IR emissivity to explain the observational
data (Draine 1994).
(9)
This is also true for the fractal grain model
(Wright 1987).

In view of the recent thoughts that the reference abundance of
the ISM (the abundances of heavy elements in both solid and
gas phases) is subsolar
(Snow & Witt 1995,
1996),
Mathis (1996,
1998)
updated the composite grain model
envisioned as consisting of three components:
(1) small silicate grains to produce the far-UV
(-1 > 6
µm-1) extinction rise;
(2) small graphitic grains to produce the 2175 Å
extinction hump; (3) composite aggregates of small silicates,
carbon, and vacuum (
45% in volume) to account for the visual/near-IR extinction.
The new composite model is able to reproduce the interstellar
extinction curve and the 10 µm silicate absorption feature.
But it produces too much far-IR emission in comparison with
the observational data
(Dwek 1997).